The present invention relates generally to magneto-optical disk drives, and more specifically to prism systems including achromatizing, circularizing and beam-splitting prisms through which a light beam is transmitted in magneto-optical systems.
In a magneto-optical disk drive, a light beam, usually a laser beam, is directed to a surface of a magneto-optical disk and reflected therefrom to a detector. Information is recorded on the surface of the disk in the form of marks or spots magnetized in a particular direction. When the light beam is reflected from the marks, the polarization of the beam will be rotated according to the magnetization of the mark. This rotation in polarization causes the marks to appear either brighter on a dark background or dark on a brighter background, depending on the sense of rotation. The reflected light beam is directed to one or more detectors capable of sensing this relative brightness, thereby providing a digital signal. Magneto-optical systems are described in Marchant, Optical Recording, pp. 68-84 (Addison-Wesley, 1990) the complete disclosure of which is incorporated herein by reference.
Prisms are utilized in magneto-optical disk drive systems for several purposes. First, because many magneto-optical systems employ semiconductor lasers which produce an elliptical laser beam, an expansion prism is used to circularize the beam. By casting the laser beam at an acute angle of incidence on a surface of the expansion prism, the beam is expanded in the plane of incidence by refraction.
Second, prisms are used in magneto-optical systems to achromatize the light beam. Fluctuations in the power supplied to the laser can cause the wavelength of the light beam to change. Changes in wavelength can change the optical path of the light beam through a prism, thereby causing the beam to deviate from the intended path to the optical disk and detectors. Achromatization is commonly accomplished by passing the light beam through a complex prism consisting of two prisms attached to one another, the prisms having different refractive indices selected such that the output angle of the light beam is independent of changes in the wavelength of the light beam. Such an achromatizing prism is described in U.S. Pat. No. 4,770,507, the complete disclosure of which is incorporated herein by reference.
Third, beam-splitting prisms are commonly used in magneto-optical systems for directing a portion of the return beam reflected from the optical disk toward the detectors. Such beam splitters commonly comprise two glass prisms adjoined along a junction surface with a semi-transparent reflective film on the junction surface between the prisms. The junction surface is disposed at an angle relative to the direction of the light beam, and allows a portion of the light beam to pass through both prisms toward the magneto-optical disk, from which it is reflected back toward the reflective surface. A portion of the return beam is then reflected from the reflective surface to the detectors.
The use of such prisms for circularizing, achromatizing and splitting the light beam in magneto-optical disk drives has created certain problems. Primary among these is misalignment of the prisms, causing inaccurate positioning of the light beam on the optical disk and/or detectors. A slight change in the angle of a prism can cause the light beam to deviate from the intended optical path. The use of multiple, independent prisms to serve the functions of circularizing, achromatizing and beam-splitting intensifies the alignment problem. In addition, known systems require a undesirably large number of separately manufactured and installed parts, increasing system cost. Moreover, the use of multiple prisms increases the size of the system beyond that desired for small form-facto disk drives.
A prism system for a magneto-optical drive is therefore desired which will perform the circularizing, achromatizing and beam-splitting functions with improved alignability, fewer parts and compact size.